There are essentially three different types of capacitive sensors. The first variation is distinguished in that the capacitance C having its sensor area is the frequency-determining element in an oscillator. A frequency shift, an amplitude change, or a damping increase is analyzed via a corresponding method. The disadvantage of this method is a very narrowly limited active region. A further disadvantage is their susceptibility to breakdown due to the influence of contamination and/or moisture.
A second type of capacitive sensor is based on a repeating charge reversal of a defined DC voltage potential, in whose capacitance-time conversion the charge reversal duration is analyzed. A disadvantage of this variation is a very narrowly limited response region. Furthermore, this principle has no static detection interval, but rather a dynamic detection interval which is a function of the approach speed and the approach angle of an object. An example of a sensor which is based on the charge reversal principle is described in a publication of EDISEN-electronic GmbH from 2001. This publication has the title “Schalten wie von Geisterhand—in der Zeitebene liegt der Zauber [Switching as if by magic—the magic is in the time plane]”. The analysis method described in this publication is the object of a European Patent Application which was published under the number EP 0 723 339 A1. A comparable approach, which is based on the charge reversal principle, is described in German Published Application DE 25 16 024. There is a German Published Application DE 198 15 324 A1, in which a sanitary valve is described that is designed so that through repeated charge reversal and determination of the charge reversal duration, the water supply may be controlled.
A third type is capacitive barriers, whose geometric positioning of the two plates corresponds to an optical light barrier and which exploit the fact that a current flows between two capacitor plates when they are powered with AC voltage. One possibility is to shield both plates from one another through a conductive and grounded object. In this case, the capacitive current becomes smaller or disappears entirely. In addition, the capacitance may be elevated if a non-grounded object is pushed between the two plates. In this case, the capacitive current becomes larger. The disadvantage of this variation is that the object must be located between the plates. A further disadvantage is the increasing susceptibility to breakdown due to external influences with increasing plate spacing. The dependence of the AC voltage amplitude on the plate spacing is also a disadvantage. A capacitive sensor for detecting the fill level of a liquid is described in German Published Application DE 199 49 985.
A further arrangement for use in the sanitary field is described in U.S. Pat. No. 5,694,653. The arrangement described allows contact-less controlling of the water supply and the temperature of the water in a washbasin. The valve is wired so that it acts as a transmitter which emits electrostatic waves. If a user moves his hand toward the valve, the hand acts as a receiver of the electrostatic radiation. A large-area receiver is positioned in the front region of the washbasin, which in turn absorbs the electrostatic radiation emitted from the body of the user. A transmission circuit is thus closed and it may be detected that the hand has approached the valve. This type of arrangement is very sensitive and has a very complex construction.